1. Field of the Invention
This invention relates to an optical pickup apparatus for reading and recording information on an optical disc and an optical disc apparatus, using the optical pickup apparatus, for reproducing and recording information on an optical disc.
2. Description of the Prior Art
An optical pickup, having a radial tilt servo, for reading information on a laser disc is known. In this prior art optical pickup, a sensor detects a tilt in the radial direction of the optical laser disc and a whole optical system of the optical pickup is mechanically tilted to compensate the tilt of the optical disc in the radial direction.
In the laser disc system, a signal is recorded with FM modulation, so that a tangential tilt of the laser disc does not largely affect the reproduction of the signal. Therefore, only the tilt in the radial direction is compensated in the laser disc system.
As mentioned above, the radial tilt servo cannot provide a high speed response because the tilt of the whole optical system is controlled, that is, the mass is too large. However, in the laser disc system, because the tilt in the radial direction is caused by a simple bend from the inner circumference to the outer circumference in the most of all cases, a high speed response is not required and is not in question now.
In an optical disc system for recording a digital signal, comprising an optical system having a high numerical aperture NA to increase a recording density, a tilt in the tangential direction of the optical disc should be compensated because the tilt in the tangential direction directly affects the reproduction quality of the recorded digital signal. This tangential tilt occurs at a higher frequency than the rotational frequency of the optical disc. Therefore, the tangential tilt compensation and the radial tilt compensation require a high frequency response.